AD9166 datasheet(65/138 Pages) AD

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Data Sheet

AD9166

Rev. 0 | Page 65 of 138

showing the simulated output return loss (S22) of the amplifier

output (see Figure 95), along with the equivalent lump element

model shown in Figure 94.

∞

–J5

–J1

–J2

J0.5

–J0.5

J0.2

–J0.2

8.91GHz

9.44 +J62.3

7.5GHz

6.81 +J39.39

10.59GHz

20.94 +J107.23

0.01GHz

49.6 –J0.18

2.24GHz

23.91 –J12.44

0.5

0.2

5.62GHz

7.77 +J17.45

4.22GHz

11.2 +J3.91

Figure 95. S22 vs. Frequency, Showing Output Impedance Relative to ZO = 50 Ω

TEMPERATURE SENSORS

The AD9166 has two junction temperature sensors, a DAC

sensor and an amplifier sensor. The amplifier sensor monitors

the temperature changes of the amplifier, and is located near the

junction of the buffer amplifier. This sensor represents the

junction temperature of the amplifier (TJ_AMP). The DAC sensor

can help monitor the temperature changes of the DAC core,

together with the digital interface inside the AD9166, and is

located near the DAC core. This sensor represents the junction

temperature of the DAC (TJ_DAC).

As the ambient temperature rises, the amplifier typically reaches

its thermal limit before the DAC core. Therefore, TJ,AMP

determines the actual maximum safe operating temperature for

both the amplifier and the DAC core inside the AD9166.

To avoid damage to the amplifier, TJ,AMP must never exceed the

limit defined in the Absolute Maximum Ratings section.

Before use, the sensors must be calibrated to remove the device

to device variations on the band gap and the circuitry that

senses the temperature. The temperature must be calibrated

against a known temperature reference to determine either the

slope or the intercept. This type of calibration is typically

referred to as one-point calibration.

The measured temperature of either sensor, TMEAS, is generally

related to the temperature code by the following formula:

TMEAS = M × CODE_x + TOFFSET

(1)

where CODE_x is the readback code at the unknown

temperature, TMEAS.

To minimize the error due to self heating during calibration, the

AD9166 must be in a low power state with only the temperature

measurement circuitry powered on.

Amplifier Junction Temperature Sensor

The junction temperature sensor reading of the amplifier is

derived from the following transfer functions:

VADC = VBGA × ((CODE_x + 4)/255)

(2)

TMEAS = 318.75 × VADC + TOS

(3)

where:

VADC is the ADC input voltage reading sampled from the

temperature sensor.

VBGA is the band gap voltage of the amplifier, with a nominal

value of 1.09 V.

CODE_x is the ADC readback code at the unknown

temperature, TMEAS.

TOS is the offset of the transfer function to be determined by

calibration

TMEAS is a temperature measurement directly proportional to the

junction temperature of the amplifier (TJ_AMP).

VBGA can be measured at the device pin, and is nominally 1.09 V

with an uncertainty of ±30 mV over process variations in

production, supply voltage variation across the specified range,

and operating temperature range. The error due to VBGA

uncertainty is ±2.75% over process, voltage, and temperature

(PVT) and the full range of the temperature sensor. Most of the

error is due to shifts in the manufacturing process, where the

remaining variations over supply voltage and operating

temperature account for only ~0.3% VBGA uncertainty.

TOS is derived by applying a known temperature, TREF, to the

device case and recording the code, CODE_REF, after the

reading has settled to a constant value within the measurement

uncertainty. During calibration, the AD9166 must be retained

in a low power state so that the error due to internal power

dissipation and self heating is minimized.

Equation 2 and Equation 3 can be combined to solve for TOS

after deriving TREF and CODE_REF during calibration.

TREF = 318.75 × VBGA × ((CODE_REF + 4)/255) + TOS

(4)

TOS = TREF − (318.75 × VBGA × (CODE_REF + 4)/255)

(5)

where:

TREF is the calibrated temperature at which the temperature sensor

is read.

CODE_REF is the readback code at the measured temperature,

TREF.

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